2020 paper 2

Question 1

Suggest and explain how the interaction between the muscles labelled in Figure 1
could cause the pupil to constrict (narrow).

Answer 1

1. Circular muscle contracts; 2. Radial muscle relaxes;

Question 2

The fovea of the eye of an eagle has a high density of cones. An eagle focuses the image of its prey onto the fovea.
Explain how the fovea enables an eagle to see its prey in detail. Do not refer to colour vision in your answer.

Answer 2

1. High (visual) acuity;
2. (Each) cone is connected to a single neurone;
3. (Cones send) separate (sets of) impulses to brain;

Question 3

Calculate the area of the fovea as a percentage of the area of the retina. The area of a circle is πr2. Use π = 3.14 in your calculation.
Show your working.
[2 marks]

Answer 3

1. Correct answer of 0.6 (%) = 2 marks;;

Question 4

The retina of an owl has a high density of rod cells. Explain how this enables an owl to hunt its prey at night. Do not refer to rhodopsin in your answer.

Answer 4

1. High (visual) sensitivity;
2. Several rods connected to a single neurone;
3. Enough (neuro)transmitter to reach/overcome threshold

Question 5

Steroid hormones are hydrophobic.

Explain why steroid hormones can rapidly enter a cell by passing through its cell-surface membrane.

Answer 5

1. Lipid soluble;

2. (Diffuse through) phospholipid (bilayer);

Question 6

In the cytoplasm, testosterone binds to a specific androgen receptor (AR). An AR is a protein.
Suggest and explain why testosterone binds to a specific AR.

Answer 6

1. Has a (specific) tertiary structure/shape; 2. (Structures are) complementary;

Question 7

The binding of testosterone to an AR changes the shape of the AR. This AR molecule now enters the nucleus and stimulates gene expression.
Suggest how the AR could stimulate gene expression.

Answer 7

1. (AR is) a transcription factor; 2. Binds to DNA/promoter;

3. (Stimulates) RNA polymerase;

Question 8

What can you conclude from the data in Table 1?

Answer 8

1. With 16 or fewer than 16 (repeats the association) is significant;
2. With 17 or more than 17 (repeats the association) is not significant;
3. With 16 or fewer than 16 (repeats) there is less than a 5% or less than 0.05 probability of being due to chance
   OR
   With 17 or more than 17 (repeats) there is more than a 5% or more than 0.05 probability of being due to chance
   OR
   Explanation of a probability value e.g. 0.30 is a 0.30 or 30% probability of being due to chance;
4. With 16 or fewer than 16 (repeats) reject the null hypothesis

Question 9

Other than the thermometer, explain how two features of the calorimeter shown in Figure 2 would enable a valid measurement of the total heat energy released.

Answer 9

1. Stirrer distributes heat (energy);
2. Insulation/space/air reduces loss/gain of heat
   OR
   Insulation/space/air reduces conduction/convection;
3. Water has high (specific) heat capacity;

Question 10

The volume of water in the calorimeter was 100 cm3
The increase in temperature recorded was 15.7 °C
4.18 J of energy are needed to increase the temperature of 1 cm3 of water by 1 °C Use this information to calculate the heat energy released in kJ per g of biomass. Show your working.

Answer 10

3.28 / 3.3 (kJ g−1) = 2 marks;;

Question 11

In natural ecosystems, most of the light falling on producers is not used in photosynthesis
Suggest two reasons why.

Answer 11

1. (Light is) reflected;
2. (Light is) wrong wavelength;
3. (Light) misses chlorophyll/ chloroplasts/photosynthetic tissue;
4. CO2 concentration or temperature is a limiting factor.

Question 12

The light absorbed by chlorophyll is used in the light-dependent reaction. Name the two products of the light-dependent reaction that are required for the
light-independent reaction.

Answer 12

1. ATP;

2. Reduced NADP;

Question 13

Calculate the number of cells in the culture after 24 hours. Give your answer in standard form.
Show your working.

Answer 13

Correct answer of 1.31/1.3 × 108

Question 14

Explain the banding pattern shown in Figure 3.

Answer 14

1. Light/I band only actin;
2. H zone/band only myosin;
3. Darkest/overlapping region actin and myosin;

Question 15

Use the information provided to describe how you could produce a calibration curve for creatinine.
Do not include details on the use of glassware in your answer.

Answer 15

1. Use (distilled) water and creatinine solution to produce dilutions (series);
2. Addition of (creatinine-)detecting solution (to each solution);
3. Using a known/specified/constant volume of a solution (e.g. diluted creatinine solution);
4. Record absorbance/transmission of solution/s using a colorimeter;
5. Plot dilution/concentration of (creatinine) solution against absorbance/transmission;

Question 16

Describe how you would determine the concentration of creatinine in a urine sample
using your calibration curve.

Answer 16

1. Use same volumes of solutions as used in producing (calibration curve)
   OR
   Add (creatinine-)detecting solution (to urine);
2. Read off (creatinine) concentration against absorbance/transmission (value) obtained;

Question 17

Describe the sequence of events involved in transmission across a cholinergic synapse.
Do not include details on the breakdown of acetylcholine in your answe

Answer 17

1. Depolarisation of presynaptic membrane;
2. Calcium channels open and calcium ions enter (synaptic knob);
   3 (Calcium ions cause) synaptic vesicles move to/fuse with presynaptic membrane and release acetylcholine/neurotransmitter;
   4 Acetylcholine/neurotransmitter diffuses across (synaptic cleft);
3. (Acetylcholine attaches) to receptors on the postsynaptic membrane;
4. Sodium ions enter (postsynaptic neurone) leading to depolarisation;

Question 18

Mutation is one cause of genetic variation in organisms. Give two other causes of genetic variation.

Answer 18

1. Crossing over;
2. Independent segregation/assortment (of homologous chromosomes);
3. Random fusion of gametes
   OR
   Random fertilisation;

Question 19

Name the relationship between the two alleles that code for flower colour.

Answer 19

Codominance

Question 20

A dwarf, pink-flowered plant was crossed with a heterozygous tall, white-flowered plant.
Complete the genetic diagram to show all the possible genotypes and the ratio of
phenotypes expected in the offspring of this cross.

Answer 20

1. ttCRCW and TtCWCW;
2. TtCRCW, TtCWCW, ttCRCW and ttCWCW ;
3. Tall pink, tall white, dwarf pink, dwarf white, and ratio 1 : 1 : 1 : 1;

Question 21

A population of this species of plant contained 9% of red-flowered plants.
Use the Hardy–Weinberg equation to calculate the percentage of pink-flowered plants in this population.
Show your working.

Answer 21

. Correct answer of 42% = 2 mark

Question 22

Calculate how many times greater the mean growth rate per day was using 37.5 g potassium nitrate than using 37.5 g ammonium sulfate.
Assume the mean biomass of the spinach plants at the start of the investigation was
0.5 g per pot.

Answer 22

1.375 / 1.3746 / 1.38 / 1.4

Question 23

Using all the information, evaluate the effect on plant growth of adding the different
fertilisers to the soil.

Answer 23

1. Potassium nitrate most effective and chicken manure least effective;
2. All fertilisers more effective than control;
3. No increase (in growth) with potassium nitrate above 30g;
4. Ammonium sulfate (shows) small/gradual increase after 30g;
5. Chicken manure effectiveness decreases after 45g
   OR
   Chicken manure effectiveness decreases at 60g;
   6 Fertiliser/s provide nitrogen source for protein;
6. No statistical test (to determine if differences are significant);
7. Only shows (results for) spinach;

Question 24

The scientist determined the dry mass of the spinach plants. First, he heated each sample at 80 °C for 2 hours.
Suggest what the scientist should do to ensure that he has removed all the water from
the sample.

Answer 24

1. Weigh and heat;

2. (Until) mass is constant;

Question 25

Suggest how AS could cause proteinuria.

Answer 25

1. Affects/damages basement membrane
   OR
   More protein channels/carriers in basement membrane;
2. Proteins can pass into the (glomerular) filtrate/tubule;

Question 26

AS results from a sex-linked mutation.

In a male with AS, where would the sex-linked mutation be located?

Answer 26

Box 4 The non-homologous section of an X 1 chromosome

Question 27

Using all the information, evaluate the use of stem cells to treat AS in humans.

Answer 27

1. Effective as D has lower protein (than B/C);
2. Not fully effective as D has higher protein than A;
3. Do not know all results for other mice in D
   OR
   Only shows results for 68% of mice;
4. Some of D mice may have been cured
   OR
   Some of D may have died;
5. Do not know actual/numerical quantity of protein;
6. (Investigation) only on mice
   OR
   (Investigation) not on humans;
7. Rejection may occur;
8. Only shows results for 20 weeks/short-time period
   OR
   Long-term effects not known;

Question 28

The scientists carried out further work to investigate how the transplanted stem cells developed after transplantation.
• The scientists transplanted stem cells from wild type male mice into AS female mice.
• After 20 weeks, they found that the quantity of protein in the urine of these female mice had significantly decreased.
• They examined cells from glomeruli in the female mice. Some of these cells contained a Y chromosome.
Suggest how the transplanted stem cells reduce proteinuria.

Answer 28

1. (Transplanted stem cells) differentiate/specialise; 2. Reduce loss of protein at the glomerulus
   OR
   Prevents protein moving into filtrate;

Question 29

Describe how enzymes could be used to insert the GH gene into a plasmid. [2

Answer 29

1. Restriction endonucleases/enzymes cuts plasmid;
   OR
   Restriction endonucleases/enzymes produces
   ‘sticky ends’;
2. Ligase joins gene/DNA and plasmid
   OR
   Ligase joins ‘sticky ends’;

Question 30

Microinjection of DNA into fertilised egg cells is a frequent method of producing transgenic fish. However, the insertion of the transferred gene into nuclear DNA may be delayed. Consequently, the offspring of transgenic fish may not possess the desired characteristic.
Suggest and explain how delayed insertion of the GH gene could produce offspring of
transgenic fish without the desired characteristic.

Answer 30

1. Cell division has occurred (before gene added);

2. (Cells producing) gametes do not receive the gene;

Question 31

Using Table 3, what can you conclude about the effectiveness of the GH gene on the growth of zebrafish

Answer 31

1. No overlap in SDs;
2. Significant increase/difference (in growth/mass)
   OR
   Increase/difference (in growth/mass) is not due to chance;

Question 32

Explain how two features of the design of this investigation helped to ensure the validity of any conclusions obtained.

Answer 32

1. Large sample size so representative;

2. 12 months so can assess/allow growth; 3. Control (present) for comparison;

Question 33

Black bears can hibernate for up to 7 months without food or water (lines 1−2).
Suggest and explain how.

Answer 33

1. Fat (store) used in respiration/metabolism;
2. Less energy/food (store) is required due to low respiration/metabolism
   OR
   Less energy/food (store) is required due less movement;
3. Gluconeogenesis;
   4.Low surface area to volume reduces heat loss
   OR
   Fat (layer/insulation) reduces heat loss;
4. Long loop of Henle so less water lost; 6. Water provided from respiration;
5. Reduced/no urination;
6. Less evaporation;

Question 34

During hibernation, the heart rate and the metabolic rate of black bears decrease (lines 3−5).
Use your knowledge of the nervous control of heart rate to describe how these are
linked.

Answer 34

1. (Lower metabolism so) less/low CO2 (in blood);
2. (Detected by) chemoreceptors;
3. (Chemoreceptors) located in aorta/medulla
   OR
   (Chemoreceptors) located in carotid artery; 4. Fewer impulses to cardiac centre;
   OR
   Fewer impulses to medulla (oblongata);
4. (More) impulses along parasympathetic/vagus pathway/neurones/nerve
   OR
   Fewer impulses along sympathetic pathway/neurones/nerve;
5. (To) SAN;

Question 35

In many mammals, ‘uncoupling proteins’ help to maintain a constant body temperature during hibernation (lines 6−7).
Suggest and explain how.

Answer 35

1. Allow passage of protons/H+; 2. (Energy) released as heat;

Question 36

Climatic change has reduced the survival rate of snowshoe hares in mountain habitats (lines 11−13).
Suggest and explain how.

Answer 36

1. Less snow so less camouflage;

2. More hares seen/eaten/killed by predators;

Question 37

Snowshoe hares within a population moult at different times (line 15).
Explain how this could ensure the survival of snowshoe hare populations in these
mountain habitats.

Answer 37

1. Hares which moult earlier (more likely to) survive;
2. Hares which moult earlier (more likely to) reproduce;
3. Pass on (advantageous) allele;
4. Frequency of allele increases (in future populations);